1. Field of the Invention
This invention relates to an electrically conductive ferrofluid composition imparted with a property for preventing electrification from occurring.
2. Prior Art
A ferrofluid or magnetic colloid is a very stable liquid in which fine particles of ferromagnetic materials such as magnetite, ferrite, iron or cobalt are finely dispersed, and the liquid itself has strong apparent magnetic properties.
Accordingly, though it takes a form of liquid, its demeanor can be freely constrained by a magnetic component such as a magnet. Thus, ferrofluids have been widely used as dampening agents, sealing agents in sealing means for magnetic discs or the like. However, when a conventional ferrofluid is used in the sealing means for some magnetic discs or the like, it has been required to provide an additional grounding means so as to remove the electrostatic charge apt to be built-up in the device. In view of this drawback, a proposal has been made to avoid such an undesirable electrostatic charge by imparting electrical conductivity to the ferrofluid itself without providing any particular grounding means. See U.S. Pat. No. 4,604,222.
This U.S. Pat. utilizes a cationic surfactant such as a quartenary ammonium salt in place of an anionic surfactant which is generally used in a ferrofluid. In the U.S. Patent, the cationic surfactant or surfactants are used to stably disperse ferromagnetic particles in a liquid carrier composed of an orgnaic solution such as mineral oil, polyalphaolefin oil or the like.
However, the above mentioned prior art utilizesd the cationic surfactant as an agent for stabilizing the dispersion and at the same time for imparting electrical conductivity. Consequently, the amount of such surfactant to be added is inevitably limited by the density of the ferromagnetic particles, namely, the amount of saturation magnetization, thus it becomes difficult to freely adjust the electrical conductivity.
In addition, a cationic surfactant is low in its thermal stability, as is well known, accordingly, there has been a problem in that the ferrofluid using suc surfactant naturally displays low thermal stability.